Bluetooth: Audio - remove superfluous check for BT_AUDIO from Kconfig

Remove the check for BT_AUDIO from the Kconfig files that had it, for
consistency (some files did not have it).

This check is not required - all Kconfig.* files are sourced from the
main Kconfig file _only_ if BT_AUDIO has been configured.  As long as
this is the case, it is not required to check in the files themselves.

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>